Abstract

The stress in the startup of uniaxial elongational flow until steady state, followed by stress relaxation, has been measured for a narrow molar mass distribution polystyrene melt with a molecular weight of . The experiments are conducted on a filament stretching rheometer, where a closed loop control of the midfilament diameter ensures controlled uniaxial extension. The closed loop control algorithm is extended to apply to the stress relaxation part of the experiment. It ensures a constant midfilament diameter, by controlling the motion of the end plates. By dividing the measured stress with the theoretically predicted stress from the Doi and Edwards model during relaxation, the stretch factors corresponding to each imposed stretch rate are obtained. These stretch factors converge to a unique envelope and eventually converge to unity for long times for all measured elongational rates.

The authors gratefully acknowledge the financial support of the Graduate School of Polymer Science from the Danish Research Training Council, and the Danish Technical Research Council to the Danish Polymer Center.